The field of "frame-based" stereotaxy is now well established. In that field, typically, a head ring is affixed to the patient's head or skull, and a localizer frame with graphic reference means is attached to the head ring. The patient is then scanned in an image scanner, such as a CT scanner, and the fiducial markings in the image scan data from the graphic reference means are used to map the image scan data or the coordinate frame of that image scanner into the coordinate reference frame of the stereotactic head ring. Following that, the patient may typically go to surgery, a calculation or determination of an anatomical target seen in the image scan data is made, and a stereotactic arc system is attached to the head ring to pass a probe to the selected anatomical target. Examples of such systems are the Radionics BRW and CRW Stereotactic Systems. A significant feature of the frame-based system is that the head frame with graphic reference means is attached to the patient's head prior to image scanning. This causes some discomfort for the patient and is an added step. It would be desirable not to have to put such a head ring on the patient's head prior to the image scan.
The field of "frameless" stereotaxy has also emerged and developed strongly in the last few years. In this situation, fiducial markings may be placed on the patient's anatomy; these may be, for example, skin markers or natural anatomical landmarks such as the tip of the nose, ears, nasion, etc. The patient is scanned in the scanner, and these physical fiducial points are identified in the image scan data. Subsequently, the patient may be brought to the operating theater and a space navigator or digitized navigator, such as a surgical operating arm or an optically coupled digitized pointer, may be used to touch each of the physical fiducial points in sequence so as to calibrate the digitized navigator relative to the physical fiducial points and thereby calibrate the digital navigator to the coordinate reference frame of the image scanner, where those fiducial points have already been identified. The advantages of frameless stereotaxy are several but include the advantage that a head ring does not have to be put on the patient's head prior to the imaging. A further advantage is the interactive nature of the digital navigators so that the surgeon can point in a direction towards the head with the digitized navigator and visualize interactively the anatomy that will be achieved in that direction according to the image scan data, which can be rendered in two dimensions or three dimensions.
There are advantages to stereotactic frames because they are stable platforms for holding surgical instruments to reach targets within the brain. On the other hand, it is the case that in most cranial operations a patient is brought to the operating room and his head is clamped to the operating table by a surgical head clamp, which is essentially a ring or semi-ring that is attached firmly by screws to the patient's skull.
It is desirable, therefore, and an object of the present invention, to bridge the gap between the frame-based and the frameless worlds of stereotaxy and provide a system and method whereby a head ring or patient attachment means need not be attached to the patient's head prior to scanning, yet a registration of the stereotactic coordinate frame can be made relative to a head clamp which is placed on the patient's head at the time of surgery (after the image scanning episode).
A further object of the present invention is to provide a system which allows or enables a digital navigator or other type of registration apparatus to make a correspondence, mapping, or registration between the coordinate frame of the image scan data (and thus of the patient's anatomy which is detected in it) and the coordinate frame of a surgical head clamp or other type of body attachment means or external apparatus which may be used on or near the patient's body at the time of the surgical intervention or treatment (but not necessarily put on the patient at the time the image scan data is taken). There are other advantages and practical objectives of the present invention which will be manifest in the description and claims below.